The present invention relates to an improved process for the preparation of ethanolamines by means of ammonolysis of ethylene oxide, and particularly to a process including the reaction of ethylene oxide with aqueous ammonia to give the mono-, di- and tri-ethanolamines.
Ethanolamines, either singly, or mixed together, are very useful products in the art. Thus, for example, tri-ethanolamine is an excellent corrosion inhibitor and mono- and di-ethanolamine, mixed together, are used in various industrial processes for the recovery of sulphuric acid and carbon dioxide from gaseous mixtures. The ethanolamines are also used as primary materials in various syntheses in the pharmaceutical industry and are reacted with fatty acids in the production of widely-used emulsifying substances.
According to the known art, ethanolamines may be prepared by ammonolysis of halohydrins or ethylene oxide, although the ethylene oxide is more widely used in commercial processes.
Thus, according to a known process, ethylene oxide and ammonia are brought into contact in a reaction chamber maintained at a temperature within the range of from 50.degree. to 275.degree. C. Normally relatively low temperatures, generally from 50.degree. to 100.degree. C., are preferred, the ethylene oxide being reacted with aqueous ammonia.
A mixture of reaction products is obtained including the mono-, di- and tri-ethanolamines, the ratio between these compounds depending on the ratio of the reagents used. For example, when excess ammonia is used with respect to the stoichiometric quantity, the reaction product is mainly monoethanolamine, while an excess of ethylene oxide favours the formation of tri-ethanolamine. In neither case is it possible, however, to obtain a single ethanolamine and, in the following description, the terms "ethanolamine" and "ethanolamines" will be understood to include the mono-, di- and tri-ethanolamines.
In known processes, the products from the reaction of ethylene oxide with ammonia are conveyed to desorption apparatus, where the excess ammonia and the greater part of the water are eliminated from the ethanolamine solution by heating at a pressure slightly greater than atmospheric.
A concentrated aqueous solution of the ethanolamines is discharged from the bottom of the desorption apparatus and conveyed to dehydrating apparatus where the residual water is removed, usually at a pressure lower than atmospheric. The ethanolamines are recovered from the bottom of the dehydrating apparatus and, if the individual components are required, these may be separated by fractional distillation.
The excess ammonia emitted at the head of the desorption apparatus is usually recovered, for example, by absorption in water in apparatus operated at a pressure about equal to that of the desorption apparatus. In fact, it is the absorption of ammonia in water to provide one of the starting materials for the reaction of the present invention which has caused some of the greatest technical problems in the prior art. This is due to the fact that the maximum concentration of ammonia absorbed in water depends not only on the pressure at which absorption is carried out but also on the absorption temperature, and in order to increase the concentration of ammonia, the solution must be cooled.
For this purpose, it is known to use an absorption column having plates provided with cooling coils. Given the small diameter of the columns needed for an ethanolamine plant of medium capacity, however, the provision of cooled plate columns of this type presents constructional difficulties and in any case such coils are not usually able to ensure homogeneous cooling of the aqueous solution. The use of filled columns of small diameter has, therefore, been proposed for the absorption of the ammonia, but the removal of heat from the solution in such apparatus, whether by external cooling jackets or by internal coils, is not readily controllable. This is because of the difficulties of heat transfer, with the small hold-up values in such columns, as well as the presence of the filling bodies themselves.
Other processes such as those in which the absorption liquid is removed from the column at an intermediate position, is cooled externally of the column and then reintroduced have also been tried and found unsatisfactory; in the particular case cited, the apparatus is complicated by the need to introduce devices for the removal of the solution from the column and for its subsequent reintroduction. Even the adoption of the system known as "pump around" for the aqueous ammoniacal solution leaving the column, has disadvantages in that it does not allow the total recovery of the ammonia, given that the gaseous phase leaving the column is in equilibrium with an aqueous solution already rich in ammonia.
According to a further known method the absorption of the ammonia is effected in an exchanger having a cooled, falling film in counter-flow with water. This solution has, among others, disadvantages typical of falling-film exchangers, such as, for example, the difficulty of achieving a homogeneous distribution of the liquid film in the exchanger tube and the necessity of operating with equi-current flows of gas and liquid, which is scarcely suitable for absorption operations.
Hence, bearing in mind that, in the processes under consideration, the absorption apparatus is connected to the desorption apparatus and is operated at a pressure slightly less than that of the latter, that is, close to atmospheric pressure, the use of absorption apparatus of any of the types described above results, inevitably, in low concentrations of the ammonia in the aqueous ammoniacal solution sent to the ammonolysis reactor. This, in turn, results in large quantities of the solution being employed since the ammonia is often required to be in excess with respect to the ethylene oxide, and hence large quantities of water vapour must later be removed in the dehydration stage, with commensurate increase in the energy used in the process.
In an alternative known method, the removal of large quantities of water in the dehydration stage is avoided by the use of concentrated aqueous solutions of ammonia, obtained by absorption at low temperatures, in the reaction with ethylene oxide. This method, however, requires the use of an expensive cooling cycle in the absorption stage.
The object of the present invention is, therefore, to provide a process for the preparation of ethanolamines by ammonolysis of ethylene oxide with aqueous ammonia and for the recovery of the said ethanolamines from the reaction products which is simple and convenient and which substantially reduces the disadvantages of the previous methods, especially those relating to the consumption of energy during the dehydration of the ethanolamines.